Light source

ABSTRACT

The present invention relates to a light emitting diode, LED, light source that may be arranged for retrofitting into a luminaire employing an incandescent light source. The light source comprises a light guide into which light from one or more LEDs in a light unit arranged at one end of the light guide is injected, and a reflector having a reflecting surface arranged at the other end of the light guide and facing towards the light guide capable of reflecting light incident on the reflecting surface. According to the present invention, the reflecting surface can be arranged in a number of exemplary ways such as to enable the light emitted from the light source to have a spatial intensity distribution that is similar to the light intensity distribution of an incandescent light source.

FIELD OF THE INVENTION

The present invention generally relates to the field of lighting design.In particular, the present invention relates to a light emitting diode(LED) light source, which may be arranged for retrofitting in luminairesemploying incandescent light sources, such as light bulbs.

BACKGROUND OF THE INVENTION

Conventional incandescent light sources generally convert an electricalcurrent to light by applying a current to a filament, typically made oftungsten, which causes the filament to glow. The filament is generallysuspended near the center of a glass bulb, thereby providing radialdistribution of light that can be used to illuminate, e.g., a room. Suchconventional incandescent light sources are typically used inchandeliers. Due to the high brightness of the glowing filament (˜1Mcd/m²), crystals in the chandelier exhibit decorative sparkling lighteffects. However, the life span of incandescent light sources istypically relatively short, usually limited to the life span of thefilament. In addition, the glass bulb generally becomes very hot due tothe high temperature of the filament, presenting a potential danger ofburning objects that come into contact with the glass bulb.

Replacing incandescent light sources with LED light sources generallyalleviates or eliminates the above problems. In addition, such areplacement provides a significant increase in the efficacy, that is theluminous flux produced by the light source as a ratio to the amount ofenergy (or power) required to produce it. However, most LEDs are onlycapable of emitting light into a hemisphere (solid angle 2π sr), whereasincandescent light sources employing a glowing filament generally emitlight uniformly into a full sphere (solid angle 4π sr).

EP1610054A2 describes a LED lamp assembly for use with automobiles, theLED lamp assembly having a central optical light guide for conductinglight emitted by a plurality of LED light sources to a deflector forprojection sideways at an angle to the axis of the light guide.

SUMMARY OF THE INVENTION

In view of the above, it is an object of the present invention toprovide a light source that alleviates or eliminates the problems asdescribed above.

This and other objectives are completely or partially achieved by alight source in accordance with the independent claim.

According to the present invention, there is provided a light sourceincluding a light unit including at least one LED, a reflector arrangedsuch that at least a portion of light incident on the reflector isreflected, and a light transmissive light guide having an input end, anoutput end, and a central region therebetween, the light guide extendingalong an axial direction. The light unit is arranged adjacent to theinput end for injecting light into the central region of the lightguide. The light guide is arranged such that its index of refraction ishigher than the index of refraction of the medium exterior to the lightguide. Furthermore, the reflector is arranged adjacent to the output endand comprises a reflecting surface facing the output end and covering atleast a portion of the output end. The reflecting surface is arrangedsuch that at least a portion of the reflecting surface is one of concaveand convex.

In the context of the present invention, by the terms “concave” and“convex” it is meant curving in, or hollowed inward, and curving out, orbulging outward, respectively.

By the configuration of the reflecting surface of such a light sourceaccording to the present invention, light emitted from the light sourcemay have a spatial intensity distribution that is substantially similarto the light intensity distribution of an incandescent light source.Furthermore, by the particular choice of the concave or convex shape ofsaid at least a portion of the reflecting surface, the light flux fromthe light source can be, e.g., substantially symmetric with respect to aplane perpendicular to the axial direction, or asymmetric, depending onthe requirements of the desired lighting application. Thus, with thepresent invention, a large variety of light sources employing LEDs maybe manufactured, each light source having light intensitycharacteristics adapted to particular user needs and/or lightingenvironment requirements.

According to an embodiment of the present invention, the light source isarranged for retrofitting into a luminaire normally employing anincandescent light source. By such a configuration, there is provided alight source that overcomes or alleviates the disadvantages ofconventional incandescent light sources, as previously described, aswell as provides a significant increase in the efficacy. Thus, thepresent invention improves the optical efficiency of conventional lightsources.

In the context of the present invention, by the term “retrofitting” itis meant fitting into a light fixture normally used for incandescentlight sources, such as a filamented light bulb, a halogen lamp, etc. Inother words, by retrofitting the light source according to the presentinvention into a luminaire normally employing an incandescent lightsource it is meant replacing the incandescent light source in theluminaire with the light source according to the present invention.

According to another embodiment of the present invention, the reflectorcomprises at least one transmitting portion arranged such that at leasta portion of light incident on the transmitting portion is transmittedthrough the reflector. According to another embodiment, the transmittingportion comprises a through hole extending along an axis. For example,the axis can be a straight axis being coincident or parallel with theaxial direction of the light guide. By these configurations, light inthe light guide is thus allowed to leave the light source by eitherpassing through the transmitting portion (or the through hole) or bybeing reflected at the reflecting surface and subsequently coupled outfrom the light guide. By these configurations, an almost viewing-angleindependent light intensity (i.e. a light intensity substantiallyindependent of the viewing angle of a user) can be achieved. Theresulting light intensity distribution is substantially similar to thelight intensity distribution of an incandescent light source. In otherwords, the light source is capable of emitting light substantiallyuniformly into a full sphere (solid angle 4π sr).

According to yet another embodiment of the present invention, the lightguide comprises a color mixing rod extending along the axial direction,wherein the color mixing rod comprises at least a portion of the centralregion of the light guide. The color mixing rod is arranged for mixinglight from multiple LEDS in the light unit and may have a hexagonalcross section. In this manner, there is provided a LED light sourcearranged such that light from a number of multiple-color LEDS is wellmixed when it reaches the output end of the color mixing rod and thus,well-mixed light can be coupled out from the light guide having anintensity distribution similar to an incandescent light source.

According to yet another embodiment of the present invention, at leastone reflecting facet is arranged on the reflecting surface such that atleast a portion of light incident on the facet is reflected. Such areflecting facet can be used to create substantial light intensityvariations as a function of the viewing angle of the user. Thus, by sucha configuration, a light source can be provided that exhibits strongviewing-angle dependent sparkling light effects (i.e. having a lightintensity that varies considerably depending on the viewing angle).

According to yet another embodiment of the present invention, thereflecting surface comprises one or more of the following: a metalcoating, such as an aluminum coating, an interference filter, such as amultilayer of thin SiO₂ and ZrO₂ layers, a diffuse coating, and aphosphor coating. The interference filter may be arranged such that itdeliberately transmits a small portion of light incident thereon. By thediffuse coatings, the brightness of the light source can be considerablyreduced, which can be desirable in some applications for improvingvisual comfort. By applying a metal coating, such as aluminum, there isachieved a relatively inexpensive, yet highly reflecting, surface.

According to yet another embodiment of the present invention, the lightsource further includes at least one translucent envelope at leastpartly surrounding the reflector. By such a configuration, the opticalperformance (that is, the light intensity distribution) or visualcomfort (for example, reduction of brightness) can be improved. The atleast one translucent envelope may comprise light scattering elements.In this manner, the brightness of the light source can be decreasedand/or the light intensity distribution of the light source can besmoothened.

It will be appreciated that such a translucent envelope can also be usedto provide a decorative enhancement in that it can be arranged so thatit hides other optical elements of the light source from the view of theuser. For example, by a suitable surface treatment, the translucentenvelope can be arranged such that it exhibits a frosted appearance, or,optionally or alternatively, the translucent envelope can be arrangedsuch that it is slightly colored by pigments dispersed in the materialof which the translucent envelope is made.

According to yet another embodiment of the present invention, the lightsource further includes a base onto which the light unit is arranged,which base includes an electrical connector arranged to mate with asocket connector of a luminaire or light fixture. The base furtherincludes electrical circuitry connected to the electrical connector,which electrical circuitry is arranged to receive electrical power fromthe electrical connector and, by means of the electrical power, operatethe light unit. In this manner, an easy fitting of the light source intoa light fixture or luminaire normally employing an incandescent lightsource is achieved. The light source may further include a heatsinkdevice arranged in the base, which heatsink device is adapted todissipate heat generated by the light unit. Thus, the surfaces of thelight source can be kept relatively cool to avoid burns to a user causedby contact with the light source. Furthermore, the life span of thelight source can be increased due to less thermal stress and/or strainin the light source components.

Other objectives, features and advantages of the present invention willappear from the following detailed disclosure, from the attached claimsas well as from the drawings.

Generally, all terms used in the claims are to be interpreted accordingto their ordinary meaning in the technical field, unless explicitlydefined otherwise herein. All references to “a/an/the [element, device,component, unit, means, step, etc]” are to be interpreted openly asreferring to at least one instance of said element, device, component,unit, means, step, etc., unless explicitly stated otherwise.

It is noted that the present invention relates to all possiblecombinations of features recited in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The above, as well as additional objects, features and advantages of thepresent invention, will be better understood through the followingillustrative and non-limiting detailed description of preferredembodiments of the present invention, with reference to the appendeddrawings, where the same reference numerals are used for identical orsimilar elements, wherein:

FIG. 1 a is a schematic view of an exemplary embodiment of the presentinvention;

FIG. 1 b is a schematic view of a portion of the exemplary embodiment ofthe present invention shown in FIG. 1 a;

FIG. 2 a is a portion of the view shown in FIG. 1 b;

FIG. 2 b is an exemplary light intensity profile of the far-fieldangular light intensity distribution of light emitted from a lightsource according to an exemplary embodiment;

FIGS. 3 a and 3 b are schematic views of yet another exemplaryembodiments of the present invention and their associated lightintensity profiles;

FIG. 4 a is a graph illustrating the shape of the reflecting surfaceaccording to exemplary embodiments of the present invention;

FIG. 4 b is an exemplary light intensity profile of the far-fieldangular light intensity distribution of light emitted from a lightsource according to an exemplary embodiment;

FIG. 5 a are schematic views of yet another exemplary embodiment of thepresent invention;

FIG. 5 b is an exemplary light intensity profile of the far-fieldangular light intensity distribution of light emitted from a lightsource according to an exemplary embodiment;

FIG. 6 a are schematic views of yet another exemplary embodiment of thepresent invention;

FIG. 6 b is an exemplary light intensity profile of the far-fieldangular light intensity distribution of light emitted from a lightsource according to an exemplary embodiment;

FIG. 7 are schematic views of yet another exemplary embodiment of thepresent invention; and

FIG. 8 is a schematic view of yet another exemplary embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Different exemplary embodiments of the present invention will now bedescribed mainly with respect to a light source being arranged forretrofitting into a luminaire normally employing an incandescent lightsource. However, it is to be understood that the light source is notlimited to the exemplary case of retrofitting applications, but mayrather be a light source used in a variety of applications.

FIG. 1 a is a schematic sectional view of a light source 1 illustratingan exemplary embodiment of the present invention, the light source 1being arranged for retrofitting into a luminaire (not shown) normallyemploying an incandescent light source, such as a filamented light bulb.Such a luminaire may also be a halogen luminaire or the like. It is tobe understood that in the context of the present invention, by the term“retrofitting” it is meant fitting into a light fixture normally usedfor incandescent light sources (that is, replacing an incandescent lightsource normally used in the luminaire with a light source according tothe present invention).

As illustrated in FIG. 1 a, the light source 1 may comprise a clearglass envelope 2, inside which a so-called remote emitter 3 is locatedthat is capable of emitting light having a spatial intensitydistribution similar to an incandescent light source, such as a glowingfilament, as will be further described in the following description.Furthermore, by the term “remote emitter” it is meant a light emittingdevice comprising one or more LEDs, a light guide, and a reflector,wherein the light emitting region of the one or more LEDs issubstantially shifted or replaced to an end of the light guide at whichthe reflector is arranged. The light source 1 may further comprise abase 4 onto which the remote emitter 3 is arranged or to which remoteemitter 3 is coupled to. The base 4 may comprise an electrical connector5, preferably threaded, which is arranged such that it is capable ofmating with a socket connector (not shown), preferably threaded, of aluminaire (not shown) employing an incandescent light source, such as afilamented light bulb.

FIG. 1 b is a schematic sectional side view of the remote emitter 3described with reference to FIG. 1 a, illustrating the idea of thepresent invention by means of a non-limiting exemplary embodiment. Asshown in FIG. 1 b, the remote emitter 3 comprises a light unit 6comprising at least one LED, in this exemplary case comprising two LEDs,arranged in a cavity 7 preferably having specularly reflecting walls.According to one example, the cavity has a cylindrical shape, althoughit is to be understood that the shape of the cavity is not limited tothis choice. It is to be understood that the present inventionencompasses embodiments having light units 6 comprising any number ofLEDs. For example, the light unit 6 may comprise a single large-sizedLED, such as an Acriche LED from Seoul Semiconductors, or a LED array,such as a LUXEON Rebel from Philips Lumileds.

The remote emitter 3 further comprises a light transmissive light guide8 having an input end 8 a, an output end 8 b, and a central region 8 ctherebetween. As illustrated in FIG. 1 b, the light unit 6 is arrangedadjacent to the input end 8 a for injecting light into the centralregion 8 c. As further illustrated in FIG. 1 b, the light guide 8 may beconfigured such that it generally extends along an axial direction.

It is also contemplated that the light guide 8 may be slightly taperedtowards the input end 8 a, i.e. the dimensions of the light guide 8 in aplane defined by the axial direction and a transversal directionperpendicular to the axial direction may become progressively largeralong the axial direction towards the output end 8 b. In the exemplarycase of the light guide 8 having a cylindrical shape, this would meanthat the diameter of the light guide 8 becomes progressively larger withthe distance along the axial direction towards the output end 8 b.

The remote emitter 3 further comprises a reflector 9 arranged adjacentto the output end 8 b, the reflector 9 being configured such that atleast a portion of light that is incident on the reflector 9 isreflected. The fraction of light reflected from the reflector 9 dependson the reflectivity of the reflector 9. The reflector 9 is furtherarranged so that it has a reflecting surface 10 that faces the outputend 8 b and covers at least a portion of the output end 8 b. Forexample, between 80% and 90% of the surface of the output end 8 b may becovered by the reflecting surface 10. Alternatively, the reflectingsurface 10 may completely cover the surface of the output end 8 b. Thereflecting surface 10 may, for example, comprise one or more of thefollowing: a metal coating, such as highly reflective aluminum, ahigh-reflectance interference filter, such as a multilayer of thin SiO₂and ZrO₂ layers, a white diffuse coating, and a phosphor coating. Theinterference filter or coating may be arranged such that it deliberatelytransmits a small portion of light incident thereon. For example, theinterference coating may be designed to transmit about 4% of the lightincident on it and (specularly) reflect the remainder of the lightincident on it. By the diffuse coatings, the brightness of the lightsource 1 can be considerably reduced, which can be desirable in someapplications for improving visual comfort. Such diffuse coatings mayalso be made transflective.

The light guide 8 may have a cylindrical shape, although the presentinvention is not limited to this particular case. On the contrary, anygeometric shape of the light guide 8 suitable for achieving thefunctions and capabilities of the light source 1 is contemplated to bewithin the scope of the present invention. The light guide 8 may be madeof a substance selected from the group of transparent polymers, polymercompounds, glass, polycarbonate, polymethylmethacrylate, acrylic, othertypes of plastics, and combinations thereof.

The operation of the remote emitter 3 is as follows.

Light emitted from the LEDs in the light unit 6 is injected(transmitted) into the central region 8 c of the light guide 8. There isgenerally no need to collimate the light before it enters the lightguide 8. It is to be understood that a small portion of the light fromthe LEDs is reflected at the light guide boundary at the input end 8 a,typically about 4%, with the rest of the light being injected into thecentral region 8 c of the light guide 8. The light in the light guide 8is then generally transported along the extension of the light guide 8towards the output end 8 b.

The light guide 8 is preferably configured such that the index ofrefraction of the light guide 8 is higher than the index of refractionof the medium exterior to the light guide 8, which exterior mediumtypically is air having an index of refraction of about 1. In otherwords, the light guide 8 is preferably arranged such that it has ahigher optical density (index of refraction) than the medium exterior tothe light guide 8. Typically, the light guide 8 is arranged such that ithas an index of refraction of about 1.5 or higher, although it is notlimited to this specific case. The transportation of light in the lightguide 8 is based on total internal reflection. Light travelling in thelight guide 8 generally does not exit the light guide 8 when it reachesthe boundary between the light guide 8 and the medium exterior to thelight guide 8, which medium is less optically dense than the light guide8, but is reflected back into the light guide 8. On one hand, when theangle of incidence of the light incident on said boundary 8 d is largerthan the critical angle (that is, the angle of incidence at which lightis refracted so that it travels along the boundary surface 8 d), thelight is reflected back without loss. On the other hand, forincreasingly lower angles of incidence, an increasingly larger fractionof the incident light will be transmitted through the boundary surface 8d out from the light guide 8.

In the context of the present invention, by the term “interior boundarysurface” it is meant the surface of the interface between the lightguide 8 and the immediate surroundings of the remote emitter 3, whichsurface has a normal facing inwards in the light guide 8.

In the context of the present invention, by the term “angle ofincidence” it is meant the angle between a ray of light incident on asurface and the normal of the surface at the point of incidence, unlessotherwise specified.

When light from the light unit 6 thus transported in the light guide 8hits the reflecting surface 10 of the reflector 9, a fraction of thelight is reflected depending on the reflectivity of the reflectingsurface 10. For example, the reflecting surface 10 may be arranged suchthat it has a reflection coefficient close to 1 or substantially 1. Bythe shape of the reflecting surface 10, most of the light reflected fromthe reflecting surface 10 is refracted at the interface between thelight guide 8 and the medium exterior to the light guide 8, namely theinterior boundary surface 8 d, and subsequently leaves the light guide8. By the particular shape of the reflecting surface 10, the intensityof the light that leaves the light guide 8 after having been reflectedat the reflecting surface 10 is substantially similar to the lightintensity of an incandescent light source, as described in furtherdetail in the following.

FIG. 2 a is a schematic sectional side view of an exemplary embodimentaccording to the present invention showing a portion of the light guide8 and the reflector 9, wherein an exemplary shape of the reflectingsurface 10 is indicated. The embodiment of the present inventionillustrated in FIG. 2 a, comprising a light guide 8 in the form of acylindrical rod, is conveniently described using the coordinates (x, z)indicated in FIG. 2 a, wherein r₀ is the radius of the base of thecylindrical rod along the x-axis, and L is the length of the extensionof the reflector along the z-axis. In this exemplary case, the z-axiscoincides with the axial direction along which the light guide 8extends.

FIG. 2 b is an exemplary light intensity profile of the far-fieldangular light intensity distribution I(θ, φ) of light leaving the lightguide 8, wherein θ is the polar angle from the z-axis and φ is theazimuthal coordinate in the xy-plane from the x-axis. The fullthree-dimensional intensity is a surface of revolution around the z-axis(in this exemplary case creating a torus around the z-axis).

The light intensity profile shown in FIG. 2 b has been produced bymodeling the embodiment of the present invention described withreference to FIG. 2 a using the illumination application softwareproduct LightTools® version 6.1.0. It should be understood that anyother light intensity profile presented in the appended drawings, whichlight intensity profile is associated with a particular embodiment ofthe present invention, has been produced in a similar manner unlessotherwise specified.

FIGS. 3 a and 3 b illustrate two exemplary embodiments of the presentinvention.

According to the embodiment illustrated in FIG. 3 a, the reflector 9 hasa reflective surface 10 facing the output end of the light guide 8. Thereflecting surface 10 thereby has a normal n directed towards the outputend of the light guide 8, as illustrated in FIGS. 3 a and 3 b. Accordingto the example illustrated in FIG. 3 a, the reflective surface 10 isarranged such that at least a portion of the reflective surface 10 isconvex, that is bulging outwards (towards the interior boundary surface8 d of the light guide 8).

According to the embodiment illustrated in FIG. 3 b, the reflectivesurface 10 is arranged such that at least a portion of the reflectivesurface 10 is concave, that is bulging inwards (away from the interiorboundary surface 8 d of the light guide 8).

Also, FIGS. 3 a and 3 b show the light intensity profiles associatedwith the embodiments described with reference to FIGS. 3 a and 3 b,respectively, which are substantially symmetric with respect to thehorizontal (x) axis, and rotationally symmetric with respect to theaxial direction (along the z-axis). These are analogous with the lightintensity profile shown in FIG. 2 b.

FIG. 4 a is a graph showing the shapes of the reflecting surfaces 10associated with the intensity profiles in FIG. 3 a (curve 11), FIG. 3 b(curve 12), and FIG. 4 b (curve 13), respectively, as projected onto thexz-plane (cf. FIG. 2 a). As can be seen from FIG. 4 b, a reflectingsurface 10 having a shape that can be described by the curve denoted bythe numeral 13 results in an asymmetric light intensity distributionwith respect to the horizontal (x) axis. L₁, L₂, and L₃ each correspondsto the length L indicated in FIG. 2 a for the embodiments illustrated inFIGS. 3 a, 3 b, and 4 b, respectively.

By means of other exemplary embodiments of the present invention, thelight intensity distribution is not limited to the light intensitydistributions as shown in FIGS. 2 b, 3 a-b, and 4 b. In this respect,FIG. 5 a illustrates another exemplary embodiment of the presentinvention, wherein the reflector 9 has a reflecting surface 10 arrangedsuch that a portion of the reflecting surface 10 (when the reflectingsurface 10 is projected onto a plane defined by the axial direction anda transversal direction perpendicular to the axial direction, in thiscase the xz-plane) is concave. It is also contemplated that thereflecting surface 10 could be arranged such that a portion of thereflecting surface 10 (when the reflecting surface 10 is projected ontoa plane defined by the axial direction and a transversal directionperpendicular to the axial direction) is convex. As further illustratedin FIG. 5 a, the reflector 9 further comprises a transmitting portion 14being arranged such that at least a portion of light incident on thetransmitting portion 14 is transmitted through the reflector 9. Inaddition to light being reflected at the reflecting surface 10 andsubsequently coupled out from the light guide 8, some of the light fromthe light guide 8 may leave the light source by passing through thetransmitting portion 14. By this configuration, an almost viewing-angleindependent light intensity can be achieved, such as illustrated in FIG.5 b that shows the light intensity profile of the far-field angularlight intensity distribution of light leaving the light guide 8associated with the embodiment illustrated in FIG. 5 a. For example, thetransmitting portion 14 may be a portion of the reflector 9 that is notcovered with a reflective material. The transmitting portion 14 may alsocomprise a through hole 14 extending along the axial direction (thez-axis). It is to be understood that the through hole 14 may also extendalong an axis that is at an angle to the axial direction. The axis alongwhich the through hole 14 extends is preferably straight, althoughthrough holes that are curved to some degree may also be contemplated.Any of the previously described embodiments and the embodimentsdescribed in the following can be combined with such a transmittingportion 14 or through hole 14.

FIG. 6 a illustrates yet another exemplary embodiment of the presentinvention, wherein the reflecting surface 10 has been provided with areflecting facet 15 arranged such that at least a portion of the lightincident on the facet 15 is reflected. Any of the light sourcesdescribed in the foregoing and in the following embodiments may comprisesuch a reflecting facet 15. Furthermore, it is to be understood thatother embodiments describing light sources comprising any suitablenumber of facets also are within the scope of the present invention.Such a facet 15 can be used to create substantial light intensityvariations as a function of the viewing angle of the user, as indicatedin FIG. 6 b, which shows the light intensity profile of the far-fieldangular light intensity distribution of light leaving the light guide 8associated with the embodiment illustrated in FIG. 6 a. By such aconfiguration, thus capable of creating substantial spatial lightintensity variations, a LED light source can be designed with enhancedviewing-angle dependent sparkling effects in a luminaire (for example, achandelier).

FIG. 7 illustrates yet another exemplary embodiment of the presentinvention, wherein the light guide comprises a color mixing rod 16extending along the axial direction (in the illustrated embodiment alongthe z-axis), for mixing light from multiple light sources in the lightunit included in the cavity 7 emitting light in general having differentcolors from each other (e.g. a cool white LED and an amber LED).According to the exemplary embodiment illustrated in FIG. 7, the colormixing rod 16 has a hexagonal cross section when viewed in the xy-plane.It is also envisaged that the color mixing rod 16 may have a squarecross section when viewed in a plane perpendicular to the axialdirection (e.g., the xy-plane). Both of these so-called hexagonal andsquare color mixing rods are very effective for mixing light of variouscolors. Preferably, the magnitude of the extension of the color mixingrod 16 along the axial direction is such that light from themultiple-color sources is well mixed when it reaches the output end ofthe color mixing rod 16, such that well-mixed light can be coupled outfrom the light guide after having been reflected from the reflector 9.It is to be understood that any one of the previously describedembodiments and the embodiments described in the following can becombined with such a hexagonal color mixing rod 16.

FIG. 8 illustrates yet another exemplary embodiment of the presentinvention, wherein any of the light sources described in the previousembodiments can be combined with one or more translucent envelopes 17 atleast partly surrounding the reflector 9. For example, the translucentenvelope 17 may comprise a hollow sphere, such as illustrated in FIG. 8.Alternatively or optionally, the light source 1 may comprise a clearglass envelope or bulb, inside which a remote emitter is arranged, suchas illustrated in FIG. 1 a. A hollow sphere, such as illustrated in FIG.8, as well as any other translucent envelope 17, can conveniently beused to improve the optical performance (that is, the light intensitydistribution) or visual comfort (for example, reduce the brightness).Optionally, the translucent envelope 17 can be provided with lightscattering elements in order to, e.g., decrease the brightness andsmoothen the light intensity distribution. Of course, the translucentenvelope 17 can also be used to provide a decorative enhancement in thatit can be arranged so that it hides other optical elements from beingviewed by the user. For example, by a suitable surface treatment, thetranslucent envelope 17 can be arranged such that it exhibits a frostedappearance, or, optionally or alternatively, the translucent envelope 17can be arranged such that it is slightly colored by pigments dispersedin the material of which the translucent envelope 17 is made, forexample a clear polymer.

The embodiments described above may be arranged such that, apart fromthe respective advantages associated therewith, each embodiment mayenable a large optical efficiency (that is, the ratio of the luminousflux outputted from the light source and the initial amount of theinstalled luminous flux). Under the assumption that each reflectingsurface 10 has a reflection coefficient of 1, and that the surface onwhich each light unit is arranged has reflection coefficient of 0, theresulting optical efficiency η for each of the embodiments depicted inFIGS. 3 a, 3 b, 4 b, 5 b, and 6 b becomes as indicated in the respectivefigures. Under these assumptions, substantially all optical losses arerelated to the optical losses at the input end 8 a of the light guide 8.

In conclusion, the present invention is related to a LED light sourcethat may be arranged for retrofitting into a luminaire employing anincandescent light source. The light source comprises a light guide,into which light from one or more LEDs in a light unit arranged at oneend of the light guide is injected, and a reflector having a reflectingsurface arranged at the other end of the light guide and facing towardsthe light guide capable of reflecting light incident on the reflectingsurface. According to the present invention, the reflecting surface canbe arranged in a number of exemplary ways such as to enable the lightemitted from the light source to have a spatial intensity distributionthat is similar to the light intensity distribution of an incandescentlight source.

The present invention has mainly been described above with reference toa few embodiments. However, as is readily appreciated by a personskilled in the art, other embodiments than the ones disclosed above areequally possible within the scope of the present invention, as definedby the appended claims.

1. A light source, comprising: a light unit including at least one lightemitting diode (LED); a reflector arranged such that at least a portionof light incident on the reflector is reflected; and a lighttransmissive light guide having an input end, an output end, and acentral region therebetween, the light guide extending along an axialdirection; wherein: the light unit is arranged adjacent to the input endfor injecting light into the central region; an index of refraction ofthe light guider is higher than the index of refraction of a mediumexterior to the light guide; the reflector is arranged adjacent to theoutput end and comprises a reflecting surface facing the output end andcovering at least a portion of the output end; and wherein: thereflecting surface is arranged such that at least a portion of thereflecting surface is either concave or convex.
 2. The light sourceaccording to claim 1, wherein the light source is arranged forretrofitting into a luminaire employing an incandescent light source. 3.The light source according to claim 1, wherein the reflector comprisesat least one transmitting portion arranged such that at least a portionof light incident on the at least one transmitting portion istransmitted through the reflector.
 4. The light source according toclaim 3, wherein the at least one transmitting portion defines a throughhole extending along an axis.
 5. The light source according to claim 4,wherein the axis is a straight axis being parallel with the axialdirection.
 6. The light source according to claim 1, wherein the lightguide comprises a color mixing rod extending along the axial direction,wherein the color mixing rod comprises at least a portion of the centralregion, the color mixing rod having a hexagonal cross section and beingarranged for mixing light from multiple LEDs in the light unit.
 7. Thelight source according to claim 1, wherein at least one reflecting facet(15) is arranged on the reflecting surface such that at least a portionof light incident on the facet is reflected.
 8. The light sourceaccording to claim 1, wherein the reflecting surface comprises one ormore of the following: a metal coating; an interference filter; adiffuse coating; and a phosphor coating.
 9. The light source accordingto claim 1, further comprising at least one translucent envelope atleast partly surrounding the reflector.
 10. The light source accordingto claim 9, wherein the at least one translucent envelope compriseslight scattering elements.
 11. The light source according to claim 1,wherein the light guide comprises a substance selected from the groupconsisting of transparent polymers, polymer compounds, glass,polycarbonate, polymethylmethacrylate, acrylic, plastic, andcombinations thereof.
 12. The light source according to claim 1, furthercomprising a base onto which the light unit is arranged, the baseincluding an electrical connector arranged to mate with a socketconnector of the luminaire, the base further including electricalcircuitry connected to the electrical connector, the electricalcircuitry being arranged to receive electrical power from the electricalconnector and, by means of the electrical power, operate the light unit.13. The light source according to claim 12, wherein the base furtherincludes a heatsink device adapted to dissipate heat generated by thelight unit.